There are a variety of applications for rectennas and tunnel diodes, including energy harvesting, rectification, and harmonic mixers for down and up conversion. Dipole, bowtie and spiral antenna shapes are some of the geometries of interest. A planar, thin film MIM diode is typically formed in a small overlap region between two (2) “arms” of an antenna. Several such devices have been developed for Radio Frequency (RF) and infrared (IR) frequency ranges. The asymmetry of the work functions of the different metals results in devices having asymmetric current flow, which allows them to be used as rectifiers and similar applications.
See the following for further background information:
Jeffrey A. Bean, et. al, “Performance Optimization of Antenna-Coupled Al/AlOx/Pt Tunnel Diode Infrared Detectors,” IEEE J. of Quantum Electronics, Vol 47, No. 1, January 2011.
Philip C. D. Hobbs, et. al., “Ni—NiO—Ni tunnel junctions for terahertz and infrared detection,” Applied Optics, Vol. 44, No. 32, Nov. 10, 2005.
U.S. Pat. No. 8,115,683 B1, “Rectenna Solar Energy Harvester.”